As the support layer of optical recording media, polymethylmethacrylate films and polycarbonate films have been conventionally employed (U.S. Pat. No. 4,614,634). Although the polymethylmethacrylate films have excellent optical characteristics, they have poor dimensional stability against the change in temperature and have poor moldability. Although the polycarbonate films are unlikely to be influenced by humidity and have good moldability, they have a drawback in that optical strain is likely to be generated.
To solve these problems, it has been proposed to employ a polyester film as the support layer (Japanese Patent Disclosure (Kokai) No. 208645/82). As the polyester film, films of polyethyleneterephthalate and its derivatives are known.
However, when a polyester film is used as the support layer or a cover layer of the optical recording media, optical strain is easily generated due to thermal stress or external force.
That is, if an external force causing bending, stretching or compression of a medium is exerted to the medium known the mechanical deformation thereof when the recording media is used, optical strain is formed thereby. When the recording medium is rotated at a high speed as in the case of an optical disk, similar deformation is usually caused by the centrifugal force. Such a deformation causes optical strain or double refraction. This phenomenon is known as photoelasticity. Since polarized laser beams are used for reading the recorded signals, photoelasticity is a major cause of reading error.
Further, if the heat resistance of the film is low, the medium may be deformed by the heat added during the processing of the recording medium or during the writing or reading of the signals. Such deformation may result in scattering or increased photoelasticity. As a result, the writing may not be conducted completely or reading error may be caused.
Further, if the surface of the film is rough, light is scattered and interfered such that the intensity of the recorded signal is significantly decreased.